Cellular or mobile networks are radio networks which may be distributed over a large geographical area. The geographical area is divided into “cells.” Each cell is served by a base station, which may serve more than one cell, and which is comprised of at least one radio transceiver, terrestrial transmission circuits, and computer processors for processing data and executing protocols and/or procedures for communicating with other base stations and with other networking equipment. For purposes of this disclosure, a cell may include all functions of the associated base station.
A base station is often fixed in location, and may comprise an evolved node B (eNB) on a Long Term Evolution (LTE) system. Together, the base stations may comprise a wireless wide area network (WWAN). The WWAN can also be communicatively coupled with a public or private network, which may include that particular aggregation of networks commonly known as the Internet.
The cellular network may comprise both “macrocells” and “small cells.” A macrocell provides radio coverage served by a high-power cellular base station, which typically has power outputs of tens of watts, and may be mounted on ground-based masts, rooftops, and other existing structures, at a height that provides a clear view over surrounding buildings and/or terrain. Small cells, on the other hand, are low-powered base stations, encompassing, for instance, femtocells, picocells, and microcells. Whereas a macrocell may have a range of a few kilometers or more, small cells generally have ranges of less than a couple kilometers (e.g., in a rural setting), and frequently within the range of a couple hundred meters or less (e.g., 10 meters within an urban setting). Mobile operators often use small cells to extend their service coverage and/or increase network capacity, for example, by offloading traffic from macrocells to small cells during peak traffic times.
User equipment, which may be mobile and moving, is configured to establish connections with the base stations of the macrocells and small cells which form the cellular network. As used herein, the term “user equipment” (UE) may refer to any type of device, including, without limitation, a mobile station, such as a mobile communication device (e.g., smart phone or other wireless phone or transceiver), tablet computer, and/or laptop computer, as well as a desktop computer. The connections formed between UEs and base stations may be established, for example, according to wireless specifications, such as LTE, Code Division Multiple Access (CDMA), Global System for Mobile Communications (GSM), Universal Mobile Telecommunication System (UMTS), or the like. Through these connections with the base stations, the UEs are able to establish voice and/or data communications with each other and other transceivers or receivers within the network or within other connected networks, including the Internet.
Small cells, such as femtocells, picocells, and microcells, may operate within, or adjacent to, the coverage area of large macrocells to form a heterogeneous network. Such a network can provide more uniform quality of broadband services across the coverage area of the macrocells. For example, small cells may compensate for macrocell radio signal degradation caused by obstruction, path loss, or interference. However, when a macrocell and nearby small cells operate on the same radio carrier, there may be co-channel interference between cells that may affect the effectiveness of the heterogeneous network. While operators may avoid such interference by allocating separate radio carriers to the macrocells and small cells, this can impact carrier utilization efficiency, and operators may be too limited in the amount of available spectrum to afford such an allocation. Therefore, interference mitigation techniques are often used when small cells are deployed.
A common technique to mitigate co-channel interference between UEs being served by a macrocell (“macrocell UE”) and UEs being served by a small cell (“small-cell UE”) is to divide a carrier's resources between the macrocell and the small cell. For example, in systems that use Orthogonal Frequency-Division Multiplexing (OFDM) based multiple access, such as LTE, some of a carrier's subcarriers may be allocated for serving UEs that are nearer to the center of the macrocell (“cell-center UEs”) and other subcarriers may be allocated for serving UEs that are nearer to the edge of the macrocell (“cell-edge UEs”). The small-cell UEs are served using the subcarriers allocated for the macrocell UEs nearer to the center of the macrocell.
In some deployments, a small cell may also use the subcarriers allocated for macrocell UEs near the edge of the macrocell, but the transmission power of the small cell UEs is restricted. Thus, the macrocell UEs nearer to the edge of the macrocell do not interfere with the small-cell UEs, and the macrocell UEs nearer to the center of the macrocell are separated by distance from the small-cell UEs. When a macrocell UE moves close to the edge of the macrocell and close to the small cell, measurement reports from the macrocell UE can trigger the macrocell to restrict the UE's resource assignments to those subcarriers allocated for macrocell UEs nearer to the edge of the macrocell.
As an illustrative example, in an LTE system, there are mechanisms which can be used to assist base stations in assigning resources for cell-center and cell-edge UEs. For instance, UEs are capable of providing a serving base station with measurement reports of the downlink signal strength of neighbor cells. These measurement reports have been commonly used in mobile systems to assist with handovers to other cells. However, for interference management, this measurement reporting capability has been expanded to allow a base station to determine whether the UE is operating within the center region of a cell or whether the UE is operating with the edge region of the cell. Triggers for downlink signal strength measurement reporting have been defined to help the macrocell to determine when a UE is crossing the boundary between the cell-center region and the cell-edge region. However, these triggers are dependent upon the UE detecting and measuring the downlink signal strength of a neighbor base station.
Using similar mechanisms, operators can also deploy multiple carriers within the coverage areas of macrocells and small cells and dynamically control the use of these carriers in order to increase spectrum efficiency. Interference can be controlled by allocating a carrier or carriers for macrocell UEs nearer to the center of the macrocell and small-cell UEs, and allocating a different carrier for macrocell UEs nearer to the edge of the macrocell's coverage area and nearer to the small cell. When the macrocell UE moves close to the macrocell edge and close to the small cell, it can be handed over to the other macrocell carrier or to the small cell in order to avoid interference with the small cell.
However. FIG. 1 illustrates a scenario in which a UE being served by a macrocell may interfere with the operation of a small cell without triggering a handover. As shown, small cell 130 operates within coverage area 122 of macrocell 120 and has unbalanced or asymmetrical uplink and downlink coverage. This imbalance between uplink coverage area 132 and downlink coverage area 134 of small cell 130 may occur, for example, it the small cell's downlink transmission power is reduced in order to control downlink interference from small cell 130. Specifically, uplink range 132 represents the area in which the small cell is able to receive an uplink signal from UE 110, and has a greater coverage area than downlink range 134, which represents the area in which the UE 110 is able to receive a downlink signal from small cell 130. While small cell 130 is shown as operating entirely within the range 122 of macrocell 120, it should be understood that the following description of an interference scenario applies to any instance in which at least a portion of the uplink range 132 of small cell 130 is within the coverage area 122 of macrocell 120.
Macrocell 120 and small cell 130 operate on one or more common radio carriers, such that UEs utilizing the macrocell, such as UE 110, and UEs utilizing small cell 130 may experience co-channel interference. For example, in the scenario illustrated in FIG. 1, UE 110 is being served by macrocell 120, and is operating on at least one radio carrier frequency that small cell 130 may utilize for the UEs being served by small cell 130. UE 110 may have been as the common carrier either as a primary cell (PCell) or secondary cell (SCell). UE 110 is within the uplink coverage area of small cell 130, but is not within the downlink coverage area of small cell 130. In other words, small cell 130 can receive uplink signals from UE 110, but UE 110 is unable to receive downlink signals from small cell 130. Consequently, UE 110's uplink signal is interfering with small cell 130, but UE 110 is unable to detect the downlink of small cell 130.
If the downlink and signals were balanced, the downlink signal strength at UE 110 could trigger a measurement event of UE 110. In the case without uplink-downlink imbalance, UE 110 would measure the downlink signal strength of small cell 130. UE 110 would then send the measurement information to macrocell 120. This transmission of measurement information would normally inform macrocell 120 about the potential interference at small cell 120 caused by UE 110. Based on the measurement information, macrocell 120 would then determine what corrective action, if any, should be taken. For example, macrocell 120 may initiate a handover procedure to hand over UE 110 to be served by small cell 130, or to continue to be served by macrocell 120 but on a different radio carrier.
However, in the scenario illustrated in FIG. 1, the downlink signal from small cell 130 is not detected by UE 110. Thus, a measurement event is not triggered. Consequently, macrocell 120 is unaware that UE 110 is causing interference for small cell 130, and therefore, unable to mitigate the interference through a corrective action, such as a handover. Small cell 130 could report the uplink interference to macrocell 120, for example, over a terrestrial connection between the two base stations, such as an X2 interface of the LTE specification. However, even if macrocell 120 is informed of the interference, it would be unable to take corrective action since macrocell 120 may be servicing multiple UEs, and the identity of the particular UE causing the interference is unknown to both macrocell 120 and small cell 130 since the interfering UE does not report the small cell downlink signal strength. Accordingly, there is a need for mechanisms to identify UEs causing uplink interference in cases where the interfering UE is not able to be identified based on signal strength measurement reports from the UE.
The LTE specification includes several mechanisms to assist base stations in coordinating the mitigation of uplink interference between cells. One mechanism is the Uplink (UL) High Interference Indication (HII) parameter. This parameter may be used by a first base station to notify a second base station about certain subcarriers of a common operating carrier that are being allocated for resource assignments for UEs near the edge of a cell of the first base station. When the second base station is a small cell base station, as in the scenario illustrated in FIG. 1, and it receives the HII parameter, it should avoid assigning the certain subcarriers to its served UEs, or should only assign the certain subcarriers to UEs transmitting with lower power.
Another LTE mechanism for interference control is the Uplink Interference Overload Indication (OI) parameter, which may be exchanged between base stations and provides uplink interference information about one of the sending base station's cells, where the particular subcarriers with an uplink, interference measurement that is above a certain threshold are identified for a particular carrier, the uplink interference measurements have been averaged over a time period, and the uplink interference is likely being caused by one of the receiving base station's cells. In some implementations, OI may be used in conjunction with HII to notify a base station when the interference levels dictated by the base station's HII parameters for a cell are not being met, such that the uplink interference levels of a cell of another base station have exceeded a threshold. An illustrative case is where a small cell is operating on the same frequency resources (e.g., subcarriers) as cell-center macrocell UEs, and one or more of these macrocell UEs are causing excessive uplink interference to the small cell. The small cell base station can report this interference to the macrocell base station using the OI parameter, and in response, the macrocell may change the boundary between its cell-center and its cell-edge regions, such that macro UEs are assigned resources from the frequency resources allocated for cell-edge UEs before they cause uplink interference to the small cell. Another illustrative example is the case where a small cell and a macrocell are operating on a common carrier, and the small-cell UEs are causing excessive uplink interference to the macrocell. The macrocell base station can report this interference to the small cell base station using the OI parameter, and the small cell may reduce the uplink transmission power of its UEs.
These current mechanisms for coordinating uplink interference mitigation between base stations by exchanging uplink interference information (i.e., identifying protected subcarriers and reporting subcarriers with excessive interference over a time period) are not suitable for resolving uplink interference in the scenario illustrated in FIG. 1. Specifically, in the illustrated scenario, there is a need to identify macrocell UE that is causing the uplink interference to small cell 130. However, a macrocell UE may be assigned different uplink sub carriers for each transmission, depending on the radio conditions at the time of assignment. Thus, identifying the subcarriers on which the interference occurs does not help to identify the interfering UE. In addition, the small cell uplink interference caused by a macrocell UE may not meet the criteria for triggering an OI report over the measurement period, but may still cause significant interference to the small cell. Thus, what is needed is to novel uplink interference indicator that may be exchanged between base stations.